TIP3055 - Base current info needed.

[Sam Jelfs]

Hi, I need to know what the minimum base current required by a tip3055 is... the issue I have is that I want to create a circuit where there is 8 transistors, the base of each controlled by a pic, with the collector controlled by a relay... diagram below, showing just one of the transistors... i need it so i can set the transistor to open, without the fuse blowing, and then when the switch is pressed and the relay closed, the fuse will blow..

NPN1 is a TIP121
NPN2 is a TIP3055
and the fuse is a 100mA quick blow.

Cheers
Sam

PS - reason for usinf a 3055 is that eventually I need to be able to draw 10A through the circuit...

 

[audioguru]

1) You forgot to look at the datasheet for the 2N3055 transistor.
Its max collector current is only 15A. The current is unlimited before the fuse will blow so instead the 2N3055 transistor will blow.

You need something to limit the current so it is less than 15A.

2) You forgot to look at the datasheet for the TIP121 darlington transistor.
Its max collector current is only 5A to 8A. The current is unlimited before the fuse will blow so it might blow before the 2N3055 transistor blows.

3) You forgot to look at the datasheet for the microcontroller. Its absolute max output current is 25mA and in this circuit it is unlimited. The microcontroller might blow before all the other stuff blows up.
You need a resistor to limit the current from the output of the microcontroller to less than 25mA.

 

[eng1]

PS - reason for usinf a 3055 is that eventually I need to be able to draw 10A through the circuit...

The relay is supposed to draw 10 A, isn't it? The transistors are used to amplify the current from the logic outputs to drive the relay's coil (you need one bjt). Is this thread related to the other one you started?

 

[audioguru]

The relay's coil is driven from a switch, not from the transistors.

Hey, the fuse was 10A, now it is only 100mA. The little fuse might blow first before the transistors blow.

A little 100mA fuse won't pass 10A for very long.

 

[eng1]

Yes, I can read a schematic... perhaps I should have said "Transistors are usually used to amplify..."

 

[Roff]

Why don't you do it like this?

 

[Sceadwian]

eng1: You'd be wrong though =) I'd say better than 99.99% of all transistors ever created are used for switching. A single modern micro processor more than makes up for several months of discrete transistor sales due to their absolutely insane transistor counts.

 

[Nigel Goodwin]

eng1: You'd be wrong though =) I'd say better than 99.99% of all transistors ever created are used for switching. A single modern micro processor more than makes up for several months of discrete transistor sales due to their absolutely insane transistor counts.

Except a transistor used as a switch is still an amplifier just the same! - it's just not a linear one!.

Small base current, large collector current - amplification!.

 

[audioguru]

The 2N3055 transistor has a wide range of spec's from not bad to terrible:
1) 10A collector current.
2) A whopping 3.3A of base current.
Its max saturation voltage is a whopping 3.0V. You cannot use a base current of 10A because its max is "only" 7A. It barely amplifies.

 

[Hero999]

Why are you even contemplating bjts? Surely this is a job for a MOSFET!

 

[Roff]

Why are you even contemplating bjts? Surely this is a job for a MOSFET!

As I posted above, why even run the fuse current through a power transistor? The fuse blows when the PIC output is high AND the switch is closed. Instead of doing this AND function in the fuse circuit, do it in the relay coil circuit. You can probably do it with a TO-92 NPN.

 

[Hero999]

Also why use a fuse when a PTC resistor is much more convenient?

 

[Roff]

Also why use a fuse when a PTC resistor is much more convenient?

I was under the impression that the goal was to blow a fuse.

 

[Hero999]

Silly me, I had forgotten that.

 

[Sceadwian]

True Nigel but that's only a technicality, the linearity curves of a switching transistor compared to one designed for amplification are completely different. Transistor built for switching make very bad amplifiers.

 

[Nigel Goodwin]

Transistor built for switching make very bad amplifiers.

But transistors built for amplifiers still make good switches - and if you're talking transistors designed as switches, they are usually specifically designed for such purposes as RF power amplifiers, line output transistors, and SMPS transistors.

 

[audioguru]

Any transistor with a low impedance voltage input and without negative feedback is very non-linear if its output level is anywhere near the supply voltage peak-to-peak.

 

[Hero999]

Most transistors are pretty non-linear, thier crappyness is normally corrected with lots of negitive feedback.

 

[Sam Jelfs]

for some reason i didn't get the emails telling me people had replied to this... don't worry about this now, change of design for other reasons has rendered this obsolete, but, just to make it a bit clearer, the fuse isn't a regular electronics fuse, but actually an "electric match" used for firing pyro's... I need to build a circuit whereby i can "arm" the channel (close the switch to that channel) but not actually fire it.... using the 3055's when i had enough power to arm the channel there was enough power to blow the pyro, with only one pyro on the channel the fire current is approx 300mA, but it's going to be much greater with 20+ pyro's on each channel... tbh i don't think ive made this any clearer, but hey, im using relays now as they do what i need of them...

Anyone have any ideas on how to use CANBus, or more to the point if i use two MCP2515's to send data between 2 locations, ie tx on one connected to rx on the other, do I need transceivers? I'm going to put another thread up in a sec on this subject...

Cheers

Sam J

 

[mneary]

If you have 20+ pyros in parallel then the monitoring will only detect if they are all open.